Fix flaky tests

[qdm12]

Since some tests fail occasionally, it might be an indicator of bad logic which could cause memory leaks and/or data races. Related to #1936 and #1968

These should be investigated. The following tests were found to fail occasionally (feel free to remove/add):

• Fix Flaky Tests in TestBlockTree_GetHashByNumber #2158
• Fix Flaky Tests in TestJson2ReadRequest #2159
• Fix Flaky Tests in Test_ext_crypto_ed25519_public_keys_version_1 #2160
• Fix Flaky Tests in TestSystemRPC #2161
• Fix Flaky Tests in lib/trie #2163
• Fix Flaky Tests in TestRuntimeChannelListener_Listen #2162
• Fix Flaky Tests in Test_ext_crypto_sr25519_public_keys_version_1 #2164
• Fix Flaky Tests in TestSync_Basic #2165
• Fix Data Races in dot/rpc/subscription Package #2166 (moved to Investigate and Fix Data Races #2147)
• Fix Flaky Tests in TestSync_SingleBlockProducer #2167
• Fix Flaky/Timing Out Tests in lib/grandpa #2168
• Fix flaky test TestHTTPServer_ServeHTTP in dot/rpc/websocket_test.go #2280
• TestHandleTransactionMessage

[EclesioMeloJunior]

I'm investigating the TestBlockTree_GetHashByNumber

[qdm12]

Our saviour has arrived

[timwu20]

@danforbes can we change this to an epic and we will will create issues for the checkboxes in the description.